The object of the present invention is to provide a method and a device for synchronizing with a reference signal marking an outside event the sampling of measuring signals through a digitizing assembly of the over-sampling type.
Such a digitizing assembly finds applications in all the fields where different measured signals are to be sampled for example, by means of several different acquisition chains by imposing that the series of samples substantially synchronize with one another. It is notably the case in the field of seismic exploration where a reference instant in relation to which propagation time intervals are counted is to be fixed for an acquisition system.
The initial reference instant that is generally selected is the instant of release of a source of seismic waves. The emitted waves are propagated in the subsoil and are received by seismic sensors distributed at the ground surface, for example. The signals delivered by these sensors are transmitted to a central control and recording laboratory, either directly or following more recent methods, by means of acquisition devices distributed in the field. Each one of them is adapted for amplifying, filtering, digitizing and storing all the signals picked up following each release of the source. After each emission-reception cycle or after a certain number of them, the stored data are centralized in the laboratory. Seismic acquisition systems are, for example, described in patents FR 2,511,772 (U.S. Pat. No. 4,583,206) or FR 2,538,194 (U.S. Pat. No. 4,628,494).
In each acquisition apparatus, the seismic signals are applied to an acquisition chain. A common acquisition chain structure comprises a fixed-gain preamplifier, a high-pass filter, an anti-aliasing low-pass filter and an analog to digital converter (A.D.C.) preceded by a variable-gain amplifier (V.G.A.) whose gain is adapted to the dynamics of the converter. The converters generally deliver 12 to 16-bit digital words which are completed by a 4-bit word giving in a binary form the gain applied to each digitized sample. This conventional structure, mostly analog, has numerous drawbacks. A variable-gain amplifier is difficult to adjust and subject to thermal variations. The cost of a stable amplifier is therefore relatively high. It is the same for the anti-aliasing filter because several switchable ones must generally be available, each one adapted to a possible sampling pitch.